Conventional fluorescent light fixtures can be found in a variety of commercial, residential and retail applications. These fixtures typically include a housing, an electronic ballast and at least one fluorescent light tube, which may be linear or curvilinear. The ballast is electrically connected to the power supply, is installed within the fixture housing, and limits the amount of current supplied to the light tube(s). In the United States, a typical ballast will be supplied with an AC voltage of approximately 120 VAC to 240 VAC. To provide appropriate starting and operating conditions, the ballast alters the frequency of the power supply from the standard mains (e.g., 60 Hz in the United States) to 20 kHz or higher, which essentially eliminates the flicker effect. Examples of conventional linear fluorescent fixtures include the T8 designation, which correlates to a tube diameter of 1 inch, the T9 designation, which correlates to a tube diameter of 1⅛ inch, and the T12 designation, which correlates to a tube diameter of 1½ inch. Each of these designations include a ballast, which may be an instant start, rapid start or programmed rapid start ballast.
Conventional fluorescent light fixtures suffer from a number of limitations, including high operating voltages, high power consumption, short life expectancy, and/or poor light output quality in terms of both color temperature and color rendering. Based upon these and other limitations, various third-parties have pursued other avenues to improve the operation of conventional fluorescent light fixtures. One alternative is replace the fluorescent lamps of conventional fluorescent light fixtures with LED lighting elements. LEDs by design are more durable, have longer life expectancy, and have superior light quality when measured in terms of color temperature and color rendering as compared to fluorescent lamps. However, the ability to replace fluorescent lamps with LEDs is hindered by operating differences between the two light sources.
Fluorescent lamps are primarily driven by electronic ballasts. These ballasts drive the lamps with alternating current (AC) signals, that are generally characterized by high frequency, high voltage with high peak voltages, and constant current with high peak currents. LED's on the other hand are low voltage direct current (DC) devices. Although they can be arrayed in series to create strings that operate at higher voltages, they still require a DC power supply to illuminate. The LEDs will only illuminate with current passing through them in one direction (DC), and, if driven with an AC power supply used to power fluorescent lamps, will only illuminate approximately half of the time when the AC signal is the correct polarity. These operating differences may require replacement of not only the fluorescent bulb itself, but also some or all of the other components of the fluorescent lighting fixture, resulting in high cost and expenditure of time.
One avenue pursued by third parties to remedy the above mentioned limitations is shown in U.S. Pat. No. 7,510,299. The patent discloses a replacement fluorescent light tube with LED arrays 22. FIG. 5 shows a first power supply circuit 100 that resides within the tube 20 and comprises an AC power source 46, a conventional ballast 48, a rectifier/filter circuit 50, a pulse width modulation (PWM) circuit 52, a pair current limiting circuits 54 and a pair of LED arrays 22, wherein a single limiting circuit 54 is associated with a single LED array 22. (Col. 3: lns. 36-41).
The '299 patent explains that: “the PWM circuit 52 receives the DC power from the rectifier/filter circuit 50 and cyclically switches the DC power on and off to the one or more current-limiting circuits 54. The DC power is switched on and off by the PWM circuit 52 at a frequency which causes the white light emitted from the LEDs 22 to appear, when viewed with a “naked” human eye, to shine continuously. The PWM duty cycle can be adjusted or varied by control circuitry (not shown) to maintain the power consumption of the LEDs 22 at safe levels.” (Col. 3, lns. 47-56). The '299 further explains that the DC power is modulated for a number of reasons: (i) to adjust the brightness or intensity of the light emitted by the LEDs 22, (ii) to improve the illumination efficiency of the light tube 20 by capitalizing upon a phenomenon in which short pulses of light at high brightness or intensity to appear brighter than a continuous, lower brightness or intensity of light having the same average power, (iii) the DC power is modulated to regulate the intensity of light emitted from the light tube 20 to compensate for supply voltage fluctuations, ambient temperature changes, and other such factors which effect the intensity of white light emitted by the LEDs 22; (iv) to raise the variations of the frequency of light above the nominal variation of 120 to 100 Hz thereby reducing illumination artifacts caused by low frequency light variations, including interactions with video screens, and (v), the DC power may optionally be modulated to provide an alarm function wherein light from the light tube 20 cyclically flashes on and off. (Col. 3, lns. 57—Col. 4, lns. 13). To accomplish these functions, the power supply circuit 100 necessitates a number of costly components, including the PWM circuit 52. In addition, the internal control signal for the PWM circuit 52 has only one possible frequency which means there can only be one brightness level. Thus, the power supply circuit 100 taught by the '299 patent suffers from a number of limitations, including but not limited to being costly to manufacture and prone to reliability and operability concerns.
The present invention seeks to overcome certain of these limitations and other drawbacks of the prior art, and to provide new features not heretofore available. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.